1. Field of the Invention
The present invention relates to a focus control method and an optical disk apparatus thereof for driving an objective lens so as to focus a light beam to a recording surface of an optical disk whereby maintaining a constant distance between the objective lens and the optical disk, especially to a focus control method and an optical disk apparatus thereof for calculating the control signal for suppressing the dislocation due to periodic disturbance, such as medium surface vibration by learning control, and executes feed-forward control in a control system to perform feedback control on a lens actuator so that the dislocation amount of the objective lens, with respect to the just focus position, becomes small (almost zero).
2. Description of the Related Art
An optical disk apparatus rotates an optical disk, such as a disk configuration and a card configuration, and radiates a light beam onto a recording surface of the optical disk to read or read/write information to and from the optical disk. As such an optical disk, compact disk (CD), digital versatile disk (DVD) and magneto-optical disk (MO) are known. A focus control is necessary for these optical disk to drive an objective lens so as to focus a light beam to a surface (recording surface) of a medium (optical disk) whereby maintaining a constant distance between the objective lens and the medium surface. Recently, it is more required that the storage capacity of the disk increase. In such a storage device, it is required to realize a smaller bit size by high Numerical Aperture (NA) and a short wave length.
However, as the depth of a focus is inversely proportional to a square of NA and is proportional to light wave length, both the high NA and the short wave length cause that the depth of a focus makes narrow. Accordingly, such a large capacity storage device is required higher accuracy of the focus follow than the conventional one.
While, in the optical disk apparatus, the surface of the disk medium vibrates toward the focusing direction (a normal line direction of the disk surface) according to the rotation of the disk due to an error of a mechanical attachment of the spindle, an error of chuck between the spindle and the disk medium, and a curvature of the disk medium itself. Such a vibration is called as ‘surface vibration’ in this invention.
For, example, if the disk is ideally plane and if the rotation axis of the disk is ideally vertical to the disk surface, the vibration of focus direction is not occurred. While, if the disk is ideally plane and if the rotation axis of the disk is not vertical to the disk surface, the surface vibration with a rotation frequency is occurred. Further, if the disk curves such as a configuration of potato chips, the surface vibration further includes two times of rotation frequency.
A focus servo system performs a feedback control for following the objective lens to the disk surface with a high accuracy by driving the focus actuator based on the focus error signal. However, if only feedback control is performed, it is obstructed to obtain a control band with a high band, because a high order mechanical resonance and a light modulated component mix to the control system as disturbances. If it is tried to expand unreasonably the control band, the problem occurs to decrease a phase margin, thereby the control system is oscillated.
Therefore, the prior art adopts a feed forward control using a learning control together with the feedback control. As the learning control is enable to improve a following ability for a periodic following target without expanding the control band, the learning control is preferable to compensate a periodic disturbance synchronized with the rotation, such as the surface vibration.
A repetitive control is known as a learning control method in focus control (for example, Japanese Laid-Open Patent H8-77589 and Japanese Laid-Open Patent H11-86309). In repetitive control, the basic period of a periodic disturbance is divided, for example, by a sampling period of the feedback control system, memory corresponding to each divided period is prepared, and the periodic disturbance is compensated.
In a disk unit, such as a magneto-optical disk unit, however, the sampling period of the feedback control system is relatively short. So if the rotation period of the disk is divided by the sampling period, memory length for learning is very long. For example, when the disk rotation frequency is 75 Hz (4500 rpm), the sampling rate is 55 kHz, and memory length is 733. If the resolution of one memory length is 256 bits, a 187 kbit capacity is required.
In order to decrease the memory length, it is possible to set the dividing period of the repetitive control system to a multiple integer of the sampling period of the feedback control system, and skip the feedback information obtained at the sampling period, but learning an accurate feed-forward compensation signal is difficult since feedback information is not effectively utilized.